In a pumping system for highly compressible fluids (for example, CO.sub.2), an important feature is the ability to keep the pump head of the pumping apparatus and the pumping fluid at a low temperature to reduce the compressibility of the pumping fluid just prior to pumping. Additionally, it is important to stabilize fluid flow by maintaining the pumping fluid at a constant temperature prior to pumping. Currently, there is intense interest in using near-critical and supercritical fluids at elevated pressures as solvents in extraction systems and in chromatographic systems. Often the solvents of interest exist as highly compressible fluids at ambient pressures and temperatures from 15-40 degrees centigrade. At ambient conditions, fluids such as carbon dioxide, ethylene, ethane and sulfur hexafluoride have high vapor pressures which significantly exceed 1 atmosphere. However, those pressures are not sufficiently high for extraction and chromatographic applications at or near supercritical conditions. Therefore, liquid state fluids must be supplied to some type of pumping system to meet pressure and flow requirements of the high pressure processes downstream of the pumping system.
The compressible fluids used for extraction or chromatography are typically supplied in the liquid phase in pressurized tanks. These tanks are not completely filled with liquid and pressure stabilizes at the vapor pressure of the liquid at the current tank ambient temperature A pressure drop in the liquid generally occurs as the liquid is supplied to the pump. Flashing may occur where fluid in the liquid phase is mixed with fluid in the gas phase resulting in a two phase fluid mixture which is more compressible than the original single phase fluid. Therefore, the efficiency and metering accuracy of a solvent delivery system can be greatly enhanced by decreasing the compressibility and hence increasing the bulk modulus of the fluid by precisely maintaining the pumping fluid at sub-ambient conditions/temperatures. It is also important to keep the temperature as stable as possible to improve the accuracy of the pumping system.
Most of the current solvent delivery systems utilize syringe pumps with pumping cylinders having large compression ratios A major drawback of such systems is the need to interrupt the chromatographic process to refill the cylinder once it is empty.
In order to decrease the compressibility of the fluid and thereby decrease the corresponding compression ratio so that liquid-type pumps with continuous flow capability can be used, it is necessary to pre-cool the compressible fluid prior to entry into the pump. Heat exchangers and cooling baths are typically employed for this purpose. Furthermore, it is also necessary to cool the pump head separately to keep the compressibility of the fluid constant at a low value during the pumping process.
A typical prior art pumping system is illustrated in FIG. 1 in which pre-cooling of the pumping fluid 1 to sub-ambient temperatures has been accomplished by feeding the pumping fluid through a heat exchanger 2 of a pre-cooler 4. The pre-cooler 4 is placed in a recirculating bath 5 which contains a cooled liquid having a regulated temperature measured by a thermocouple 6. The cooled liquid is also circulated to a pump head 7 by a recirculating pump 8 to keep it at sub-ambient temperatures. FIG. 2 is a more detailed view of the pump head and illustrates problems associated with cooling the pump head and the pumping fluid separately.
Another prior art technique for cooling a pump head is disclosed in co-pending patent application Ser. No. 07/662,687 entitled "Cooled Pumping System" in which a single source of cryogenic fluid is employed for cooling a pump head, having a much higher thermal conductivity than the pump body, simultaneously with the pumping fluid just prior to pumping.
Thermoelectric devices which utilize the Peltier effect have come into widespread use as solid-state heating and cooling elements. However, the amount of heat such devices are capable of removing is limited by the inherent ability to transfer heat from the outer surfaces of the devices. Additionally, the efficiency of such devices degrades rapidly with the temperature differential between the surface being cooled and the surface of the heat exchanger. Prior to the invention, it was not possible to effectively thermoelectrically cool a liquid such as CO.sub.2 so that it can by pumped at sub-ambient conditions.
It is desirable to eliminate the need for either recirculating baths or cryogenic cooling. Both require constant maintenance and in the case of cryogenic cooling, lots of cryogenic cooling agent, usually CO.sub.2, which comes in bulky tanks.